The vastus lateralis (s/ or s/), also called the ''vastus externus''[citation needed] is the largest and most powerful part of the
quadriceps femoris, a muscle in the
thigh. Together with other muscles of the quadriceps group, it serves to extend the knee joint, moving the lower leg forward. It arises from a series of flat, broad
tendons attached to the
femur, and attaches to the outer border of the
patella. It ultimately joins with the other muscles that make up the quadriceps in the
quadriceps tendon, which travels over the knee to connect to the
tibia. The vastus lateralis is the recommended site for
intramuscular injection in infants less than 7 months old and those unable to walk, with loss of muscular tone.[1]

Structure

The vastus lateralis muscle arises from several areas of the
femur, including the upper part of the
intertrochanteric line; the lower, anterior borders of the
greater trochanter, to the outer border of the
gluteal tuberosity, and the upper half of the outer border of the
linea aspera. These form an
aponeurosis, a broad flat tendon which covers the upper three-quarters of the muscle. From the inner surface of the aponeurosis, many muscle fibres originate. Some additional fibres arise from the tendon of the
gluteus maximus muscle, and from the
septum between the vastus lateralis and short head of the
biceps femoris.

The fibers form a large fleshy mass, attached to a second strong aponeurosis, placed on the deep surface of the lower part of the muscle. This lower aponeurosis becomes contracted and thickened into a flat tendon that attaches to the outer border of the
patella, and subsequently joins with the quadriceps femoris tendon, expanding the capsule of the
knee-joint.